Positron Emission Tomography (PET) imaging provides high resolution and quantitation from the PET images. Peptides or other small molecules can be labeled with the positron emitters 18F, 64Cu, 66Ga, 68Ga, 76Br, 94mTc, 86Y, and 124I to name a few. The positron emitted from the nucleus of the isotope is ejected with different energies depending on the isotope used. When the positron reacts with an electron two 511 keV gamma rays are emitted in opposite directions. The energy of the ejected positron controls the average distance that a positron travels before it is annihilated by hitting an electron. The higher the ejection energy the further the positron travels before the collision with an electron. A low ejection energy for a PET isotope is desirable to minimize the distance that the positron travels from the target site before it generates the two 511 keV gamma rays that are imaged by the PET camera. Many isotopes that emit positrons also have other emissions such as gamma rays, alpha particles or beta particles in their decay chain. It is desirable to have a PET isotope that is a pure positron emitter so that any dosimetry problems will be minimized.
The half-life of the isotope is also important, since the half-life must be long enough to attach the isotope to a targeting molecule, analyze the product, inject it into the patient, and allow the product to localize, clear from non-target tissues and then image. If the half-life is too long the specific activity may not be high enough to obtain enough photons for a clear image and if it is too short the time needed for manufacturing, commercial distribution and biodistribution may not be sufficient. F-18 (β+ 635 keV 97%, t1/2 110 min) is one of the most widely used PET emitting isotopes because of its low positron emission energy, lack of side emissions and suitable half-life. The F-18 is produced with a high specific activity. When an isotope is attached to a molecule for targeting it is usually accompanied by some unreacted targeting agent, which is often present in a large molar excess compared to the radiolabeled product. Usually, the labeled product and the unlabeled product can compete for the same target in-vivo so the presence of the cold targeting agent lowers the effective specific activity of the targeting agent. If the F-18 is attached to a molecule which has a very high uptake such as 2-fluoro-2-deoxy glucose (FDG) then effective specific activity is not as important. However, if one is targeting a receptor with a labeled peptide or performing an immunoPET pretargeting study with a limited number of binding sites available, the cold targeting agent could potentially block the uptake of the radiolabeled targeting agent if the cold targeting agent is present in excess.
Conventional F-18 labeling of peptides involves the labeling of a reagent at low specific activity, HPLC purification of the reagent and then conjugation to the peptide of interest. The conjugate is often repurified after conjugation to obtain the desired specific activity of labeled peptide. An example is the labeling method of Poethko et al. (J. Nucl. Med. 2004; 45: 892-902) in which 4-[18F]fluorobenzaldehyde is first synthesized and purified (Wilson et al, J. Labeled Compound and Radiopharm. 1990; XXVIII: 1189-1199) and then conjugated to the peptide. The peptide conjugate is then purified by HPLC to remove excess peptide that was used to drive the conjugation to completion. The two reactions and purification would not be a problem if F-18 had a long half-life. However the half-life of F-18 is only 2 hr so all of the manipulations that are needed to attach the F-18 to the peptide are a significant burden.
These methods are tedious to perform and require the use of equipment designed specifically to produce the labeled product and/or the efforts of specialized professional chemists. They are not kit formulations that could routinely be used in a clinical setting. A need exists for a rapid, simple method of 18-F-labeling of targeting moieties, such as proteins or peptides, that results in targeting constructs of suitable specific activity and in vivo stability for detection and/or imaging, while minimizing the requirements for specialized equipment or highly trained personnel and reducing operator exposure to high levels of radiation. A further need exists for prepackaged kits that could provide compositions required for performing such novel methods.